How to get this code to work with negative numbers Java

I have a function for evaluating equations in the form of strings but I don’t think (I didn’t write it and I am not good enough at Java to understand it yet) that it works with negative numbers. Is there either a better way to implement this same functionality (evaluating equations in the form of strings and also having trig functions) or a way to get the code to work with negatives? This is all in Java.

    public static twovars eval(final String str) {
    return new Object() {
        int pos = -1, ch;
        void nextChar() {
            ch = (++pos < str.length()) ? str.charAt(pos) : -1;
        }

        boolean eat(int charToEat) {
            while (ch == ' ') nextChar();
            if (ch == charToEat) {
                nextChar();
                return true;
            }
            return false;
        }

        twovars parse() {
            nextChar();
            twovars b = parseExpression();
            double x = b.getDub();
            if (pos < str.length()) throw new RuntimeException("Unexpected: " + (char)ch);
            return new twovars(b.getBool(), x);
        }

        // Grammar:
        // expression = term | expression `+` term | expression `-` term
        // term = factor | term `*` factor | term `/` factor
        // factor = `+` factor | `-` factor | `(` expression `)`
        //        | number | functionName factor | factor `^` factor

        twovars parseExpression() {
            twovars b = parseFactor();
            double x = b.getDub();
            for (;;) {
                if      (eat('+')) x += parseTerm().getDub(); // addition
                else if (eat('-')) x -= parseTerm().getDub(); // subtraction
                else return new twovars(false, x);
            }
        }

        twovars parseTerm() {
            twovars b = parseFactor();
            double x = b.getDub();
            for (;;) {
                if      (eat('*')) x *= parseFactor().getDub(); // multiplication
                else if (eat('/')) {
                    if(parseFactor().getDub() != 0) x /= parseFactor().getDub();
                    else return new twovars(true, 0);
                } // division
                else return new twovars(false, x);
            }
        }

        twovars parseFactor() {
            if (eat('+')) return new twovars(false, parseFactor().getDub()); // unary plus
            if (eat('-')) return new twovars(false, -parseFactor().getDub()); // unary minus
            double x;
            int startPos = this.pos;
            if (eat('(')) { // parentheses
                x = parseExpression().getDub();
                eat(')');
            } else if ((ch >= '0' && ch <= '9') || ch == '.') { // numbers
                while ((ch >= '0' && ch <= '9') || ch == '.') nextChar();
                x = Double.parseDouble(str.substring(startPos, this.pos));
            } else if (ch >= 'a' && ch <= 'z') { // functions
                while (ch >= 'a' && ch <= 'z') nextChar();
                String func = str.substring(startPos, this.pos);
                x = parseFactor().getDub();
                if (func.equals("sqrt")) x = Math.sqrt(x);
                else if (func.equals("sin")) x = Math.sin(Math.toRadians(x));
                else if (func.equals("cos")) x = Math.cos(Math.toRadians(x));
                else if (func.equals("tan")) {
                    if(x % Math.PI == 0) x = Math.tan(Math.toRadians(x));
                    else return new twovars(true, x);
                }
                else if (func.equals("csc")) x = 1/Math.sin(Math.toRadians(x));
                else if (func.equals("sec")) x = 1/Math.cos(Math.toRadians(x));
                else if (func.equals("cot")) x = 1/Math.tan(Math.toRadians(x));
                else if (func.equals("arcsin")) {
                    if (x>1 || x<1) x = Math.asin(Math.toRadians(x));
                    else return new twovars(true, x);
                }
                else if (func.equals("arccos")) {
                    if(x>1 || x<1) x = Math.acos(Math.toRadians(x));
                    else return new twovars(true, x);
                }
                else if (func.equals("arctan")) x = Math.atan(Math.toRadians(x));
                else if (func.equals("log")) {
                    if(x>0) x = Math.log(x);
                    else return new twovars(true, x);
                }
                else throw new RuntimeException("Unknown function: " + func);
            } else if (ch >= 'A' && ch <= 'Z') { // named constants
                while (ch >= 'A' && ch <= 'Z') nextChar();
                    String s = str.substring(startPos,this.pos);
                    if (s.equals("PI")) {
                        x = Math.PI;
                    } else if (s.equals("E")) {
                        x = Math.E;
                    } else {
                        throw new RuntimeException("Invalid constant: "+s);
                    }
                } else {
                    throw new RuntimeException("Unexpected: " + (char)ch);
                }

            if (eat('^')) x = Math.pow(x, parseFactor().getDub()); // exponentiation

            return new twovars(false, x);
        }
    }.parse();